The invention relates generally to processes for electrographic image development. More specifically, the invention relates to apparatus and methods for electrographic image development, wherein the image development process is optimized by setting the average developer bulk velocity with reference to the imaging member velocity.
Processes for developing electrographic images using dry toner are well known in the art and are used in many electrographic printers and copiers. The term xe2x80x9celectrographic printer,xe2x80x9d is intended to encompass electrophotographic printers and copiers that employ a photoconductor element, as well as ionographic printers and copiers that do not rely upon a photoconductor. Electrographic printers typically employ a developer having two or more components, consisting of resinous, pigmented toner particles, magnetic carrier particles and other components. The developer is moved into proximity with an electrostatic image carried on an electrographic imaging member, whereupon the toner component of the developer is transferred to the imaging member, prior to being transferred to a sheet of paper to create the final image. Developer is moved into proximity with the imaging member by an electrically-biased, conductive toning shell, often a roller that may be rotated co-currently with the imaging member, such that the opposing surfaces of the imaging member and toning shell travel in the same direction. Located adjacent the toning shell is a multipole magnetic core, having a plurality of magnets, that may be fixed relative to the toning shell or that may rotate, usually in the opposite direction of the toning shell.
The developer is deposited on the toning shell and the toning shell rotates the developer into proximity with the imaging member, at a location where the imaging member and the toning shell are in closest proximity, referred to as the xe2x80x9ctoning nip.xe2x80x9d In the toning nip, the magnetic carrier component of the developer forms a xe2x80x9cnap,xe2x80x9d similar in appearance to the nap of a fabric, on the toning shell, because the magnetic particles form chains of particles that rise vertically from the surface of the toning shell in the direction of the magnetic field. The nap height is maximum when the magnetic field from either a north or south pole is perpendicular to the toning shell. Adjacent magnets in the magnetic core have opposite polarity and, therefore, as the magnetic core rotates, the magnetic field also rotates from perpendicular to the toning shell to parallel to the toning shell. When the magnetic field is parallel to the toning shell, the chains collapse onto the surface of the toning shell and, as the magnetic field again rotates toward perpendicular to the toning shell, the chains also rotate toward perpendicular again. Thus, the carrier chains appear to flip end over end and xe2x80x9cwalkxe2x80x9d on the surface of the toning shell and, when the magnetic core rotates in the opposite direction of the toning shell, the chains walk in the direction of imaging member travel.
The prior art indicates that it is preferable to match developer linear velocity to the imaging member velocity. Prior art printers have attempted to relate the velocity of the developer to the velocity of the imaging member by measuring the surface velocity, or linear velocity, of the developer, based on high speed camera measurements of the velocity of the ends of the carrier chains. This invention, however, is based on the surprising recognition that such measurements based on linear velocity greatly overestimate the actual developer velocity, thereby causing a substantial mismatch in velocity of the developer and imaging member. This overestimation results from a focus on the surface of the developer nap, i.e., the ends of the carrier chains, because as the carrier chain rotates from parallel to the toning shell to perpendicular to the toning shell, the ends of the carrier chains accelerate, causing the surface of the developer nap to appear to move at a higher velocity than the greater volume of the developer. While mismatched developer and imaging member velocities may produce adequate image quality for some applications, as the speed of image production increases, mismatched developer bulk and imaging member velocities may lead to image quality problems. Accordingly, it is an object of the present invention to provide an electrographic printer in which the average developer bulk velocity is about the same as the imaging member velocity.
The present invention solves these and other shortcomings of the prior art by providing a method and apparatus for generation of electrographic images in which the average developer bulk velocity is within preferred ranges relative to the imaging member velocity. In one embodiment, the invention provides an electrographic printer, including an imaging member moving at a predetermined velocity, a toning shell located adjacent the imaging member and defining an image development area therebetween, and a multipole magnetic core located adjacent the toning shell, wherein developer is caused to move through the image development area in the direction of imaging member travel at an average developer bulk velocity greater than about 37% of the imaging member velocity. In another embodiment, the average developer bulk velocity is greater than about 50% of the imaging member velocity. In a further embodiment, the average developer bulk velocity is greater than about 75% of the imaging member velocity. In a yet further embodiment, the average developer bulk velocity is greater than about 90% of the imaging member velocity. In a still further embodiment, the average developer bulk velocity is between 40% and 130% of the imaging member velocity, and preferably between 90% and 110% of the imaging member velocity. In another embodiment, the average developer bulk velocity is substantially equal to the imaging member velocity. In yet another embodiment, the electrographic printer includes a cylindrical magnetic core or other configuration of magnetic field producing means that produces a magnetic field having a field vector in the toning nip that rotates in space.
A further embodiment is a method for generating electrographic images, the method including providing an electrographic printer comprising an imaging member moving at a predetermined velocity, a toning shell located adjacent the imaging member and defining an image development area therebetween, and a multipole magnetic core located inside the toning shell, and causing developer to move through the image development area in the direction of imaging member travel at an average developer bulk velocity greater than about 37% of the imaging member velocity. In a further embodiment, the average developer bulk velocity is greater than about 50% of the imaging member velocity. In another embodiment, the average developer bulk velocity is greater than about 75% of the imaging member velocity. In a further embodiment, the average developer bulk velocity is greater than about 90% of the imaging member velocity. Preferably, the average developer bulk velocity is between about 40% and about 130% of the imaging member velocity, and more preferably between about 90% and about 110% of the imaging member velocity. In a still further embodiment, the average developer bulk velocity is substantially equal to the imaging member velocity.
An additional embodiment provides an electrographic printer including an imaging member moving at a predetermined velocity, a toning shell located adjacent the imaging member and defining an image development area therebetween, and a multipole magnetic core located adjacent the toning shell, wherein developer is caused to move through the image development area in the direction of imaging member travel at an average developer bulk velocity wherein the developer flow in gm/(in. sec.) divided by the developer mass area density in gm/in2 is greater than about 37% of the imaging member velocity. In a further embodiment, the developer is caused to move through the image development area in the direction of imaging member travel at an average developer bulk velocity wherein the developer flow in gm/(in. sec.) divided by the developer mass area density in gm/in2 is between about 90% and 110% of the imaging member velocity.
An additional embodiment provides an electrographic printer including an imaging member moving at a predetermined velocity, a toning shell located adjacent the imaging member and defining an image development area therebetween, and a multipole magnetic core located adjacent the toning shell, wherein developer is caused to move through the image development area in the direction of imaging member travel at a rate with excess free volume in the image development area to be between about 7% and about 93%, preferably between about 25% and about 75%, and more preferably about 50%. In another embodiment, the fraction of excess free volume is determined by the equation VF=1xe2x88x92(kNTVT+NCVC)/(fL), wherein k is between about 0.0 and about 1.0. In yet another embodiment, the fraction of excess free volume is determined by the equation VF=1xe2x88x92(kNTjVC+NCVC)/(fH), wherein k is between about 0.0 and about 1.0 and j is between VT/VC and 1.0.
An additional embodiment provides a method for generating electrographic images including providing an electrographic printer comprising an imaging member moving at a predetermined velocity, a toning shell located adjacent the imaging member, and defining an image development area therebetween, and a multipole magnetic core located inside the toning shell and causing developer to move through the image development area in the direction of imaging member travel at an average developer bulk velocity such that there is substantially no relative motion in the process direction of the developer with reference to the imaging member, wherein the developer is caused to move in a direction normal to the direction of developer bulk flow.